Chopper type inverters are known in which semiconductor switches are connected in a bridge across the positive and negative sides of a DC source and controlled to provide selectively variable frequency and variable voltage polyphase power to a load such as an AC motor, but known chopper type inverters require a filter between the chopper and the power bridge to isolate the inverter from the DC source during transient operating conditions of the motor.
Pulse width modulated (PWM) inverters are known wherein the carrier ratio (i.e., carrier frequency to inverter fundamental frequency) remains constant over the operating range of the inverter, but many inverters are variable ratio synchronized carrier systems wherein the carrier steps through a sequence of ratios and switch points as the operating frequency is increased, thereby permitting a wider range of inverter output voltage, perhaps in the order of 10:1.
Known chopper type inverters having feedback diodes in shunt to the inverter semiconductor switches require a filter between DC source and the inverter bridge in order to reduce motor current ripple and effect a stable system. The motor current path is interrupted when the inverter swtiches are open, and when no filter is provided, the voltage at the motor terminals is indeterminate. The motor current decays rapidly and may be discontinuous so no current is flowing in the motor, particularly during low excitation. Such open motor current paths may result in high ripple and low-frequency harmonics in the motor current at low motor speeds, and this may cause overheating of the motor and instability in the drive system when no filter is utilized. However, if a filter is provided, the filter voltage varies in accordance with the motor voltage, and accordingly when the motor is running at low speed, the low filter voltage, in effect, provides a low impedance path for the motor current so that motor current flow is substantially continuous, ripple in the motor current at low motor speed is reduced, and system stability is improved. However, such filter is expensive, and the inductance and capacitance of the filter in known inverters introduces a time lag which prevents fast system response and also may, in itself, lead to motor instability.
In known inverters with forced commutation, the voltage across the commutation capacitors increases in proportion to the magnitude of the inverter output current, and this can result in overcharge on the commutation capacitors as well as commutation failure within the inverter.